The purpose of this research is to test the hypothesis that accessory pathways are a cause of supraventricular tachycardia in some patients who have reentrant rhythm disorders but who do not have the classical electrocardiographic manifestations of the Wolff-Parkinson-White Syndrome. Patients who have reentrant supraventricular tachycardia but who do not have ventricular preexcitation (a delta wave on the surface electrocardiogram) will be studied by electrode cathether techniques to determine the prevalence of accessory pathways in this population. Patients who are found to have reentrant tachycardia involving an accessory pathway which conducts only from the ventricle to the atrium in whom medical therapy has been ineffective will be considered candidates for surgical treatment. Intra-operative epicardial mapping and surgical division of the accessory pathway with abolition of the tachycardia will constitute proof of the hypothesis. Clinical data from patients who have accessory pathways demonstrated by electrode catheter and operative techniques will be compared to clinical data collected from patients who have atrioventricular nodal reentry or other supraventricular tachycardias to develop clinical guidelines to predict which patients with supraventricular arrhythmias might benefit from further study and surgical therapy.